Not applicable.
Not applicable.
1. Field of the Invention
This invention relates generally to conductive contacts for electrical components and more particularly to electrical contacts adapted to receive solder balls.
2. Background of the Invention
Modem electronics require reliable, low profile, high density electrical interconnections. One technique used to connect integrated circuits and other electronic components to a substrate, such as a printed circuit board, utilizes conductive spheres in the form of solder balls. For example, integrated circuits using ball grid array (BGA) technology include an array of solder balls attached to conductive pads on the bottom of the package. During manufacture of the integrated circuit, a solder reflow process is used to attach the solder balls to the conductive pads on the package which are sometimes coated with a solder wettable material such as gold or tin/lead. For example, gold is plated onto a copper base metal in a Tessera xcexcBGA(copyright) package, as described in xe2x80x9cPlacement and Reflow of 0.3 mm Diameter Solder Balls for Chip-Scale xcexcBGA(copyright) Devices,xe2x80x9d Chip Scale Review, Dec. 1997, pages 28-35. A further solder reflow process is used attach the solder balls on the component package to conductive pads on the printed circuit board.
Some electronic components which utilize solder ball connections include electrically conductive contacts or conductors, each having a contact end for receiving a solder ball. One such component is an electrical connector, as described in U.S. Pat. No. 6,152,747 entitled ELECTRICAL CONNECTOR. A portion of a solder ball contact 10 of the type described in U.S. Pat. No. 6,152,747 is shown in FIG. 1 to include a solder ball contact end 18 coupled to an elongated portion 14 by a curved interconnect 16. A further contact end (not shown) is provided at a second end of the elongated portion 14 for electrically interconnecting to a further component.
In manufacture, a solder ball 20 is positioned on the contact end 18, which may be dimpled to provide a concave surface, and a reflow process is used to melt the solder ball, causing the solder ball to adhere to the contact end. Ideally, the solder ball 20 is positioned in the center of the contact end, as shown by the dotted solder ball outline 22. However, during the reflow process, solder may flow beyond the contact end 18, as shown by the solid solder ball outline 24. This phenomena can result in the solder balls of adjacent contacts having different heights, thereby impairing the reliability of the resulting interconnections. Further, because the solder balls are not precisely located relative to the contact end 18, interconnection density and/or reliability may be adversely impacted.
A U.S. Pat. No. 6,079,991 entitled METHOD FOR PLACING CONTACT ON ELECTRICAL CONNECTOR describes an electrical connector having a solder ball mounting interface with high coplanarity. The connector includes contacts secured to an insulative housing having recesses into which a portion of a respective contact extends. A controlled volume of solder paste and a solder ball are positioned in each recess. The structure is then heated, causing the solder paste and solder ball to fuse to the contact end extending into the recess. The recesses are substantially uniform in size and shape and can receive a highly uniform amount of solder paste. Also, the recesses locate the position of each solder ball in the X, Y, and Z directions. A contact tab area which includes, but extends beyond, the contact end to which a solder ball is attached, is plated with a solder receptive material such as gold, tin or tin alloys. Solder resist areas are provided on central portions of the contacts to prevent solder wicking.
Electrical contacts having contact ends in the form of pins for soldering or press-fit attachment to plated vias of a printed circuit board or contact tails for surface mount attachment to conductive pads of a printed circuit board are sometimes coated with gold in order to prevent an oxide build up. Oxide has an insulative effect and thus, degrades contact performance. Further, gold advantageously reduces contact resistance.
It is an object of the invention to provide an electrical contact having a contact end for receiving a solder ball at a precise, predetermined position.
It is a further object of the invention to provide an electrical component having a plurality of solder ball contacts with the solder balls being at a substantially uniform height with respect to the component.
The foregoing and other objects are achieved with an electrical contact having an elongated portion and a contact end coupled to the elongated portion. The contact end has a contact surface adapted to receive a conductive sphere, such as a solder ball, and edge surfaces adjacent to the contact surface adapted to contain the solder ball on the contact surface. The contact surface is comprised of a solder-wettable material and the edge surfaces contain the solder ball in place by mechanical and/or material characteristics. In one embodiment, an edge surface of the contact end adjacent to the elongated portion of the contact is comprised of a less solder-wettable material.
Suitable materials for the contact surface include gold, palladium, and tin/lead alloys and suitable materials for the edge surfaces include copper alloys and nickel. In a preferred embodiment, the contact surface is gold and the edge surfaces are beryllium copper.
With this arrangement, the solder ball adheres to the contact surface and not to the adjacent edge surfaces, resulting in precise alignment of the solder ball relative to the contact. For electrical components having an array of such contacts, this precise alignment results in the solder balls being at a substantially uniform height and more reliable spacing between adjacent solder balls as is particularly important in high density components.
Also described is an electrical contact having a curved lead and a contact end connected to the curved lead. The contact end has a surface comprising a first region of a relatively high solder wettability and a second region of a lower solder wettability adjacent to the curved lead.
An electrical connector according to the invention includes a plurality of contacts, each having an intermediate portion, a first contact end at a first end of the intermediate portion, and a second contact end at a second end of the intermediate portion. The first contact end has a gold surface adapted to receive a solder ball and an edge surface adjacent to the gold surface comprised of a less solder-wettable material than gold. An insulative housing supports the contacts.
A method of fabricating a conductive contact having a contact end adapted to receive a conductive sphere includes plating a sheet of metal with at least one stripe of a solder-wettable material and stamping the plated sheet to define at least a portion of the contact end in the gold stripe. Since the metal sheet is plated and then the contact end is stamped, edge surfaces of the contact end orthogonal to the contact surface are not plated. These edge surfaces are comprised of the contact metal, such as beryllium copper, which is less solder-wettable than gold.